The invention relates to a receiving method in a data transmission system in which a CDMA multiple access method is applied and a decorrelating detector is used in the detection of a received signal.
CDMA (Code Division Multiple Access) is a multiple access method based on a spread spectrum technique, and it has recently been put into use in cellular radio systems in addition to earlier developed FDMA (Frequency Division Multiple Access) and TDMA (Time Division Multiple Access Methods). CDMA has many advantages over the earlier developed methods, such as simplicity of frequency planning, and spectrum efficiency.
In a CDMA multiple access method, a narrow-band data signal of a user is multiplied to a relatively broad band by a spreading code having a much broader band than the data signal. Band widths used in known test systems include e.g. 1.25 MHz, 10 MHz and 25 MHz. The multiplication spreads the data signal over the entire band to be used. All the users transmit simultaneously on the same frequency band. On each connection between a base station and a mobile station, a different spreading code is used, and the signals of the users can be distinguished from one another in the receivers on the basis of the spreading code of the user. If possible, the spreading codes are selected in such a way that they are mutually orthogonal, i.e. they do not correlate with one another.
Correlators or matched filters in CDMA receivers implemented in a conventional manner synchronize with the desired signal, which is identified on the basis of the spreading code. In the receiver, the data signal is restored to the original band by multiplying it by the same spreading code as was used in the transmission step. Ideally, the signals that have been multiplied by some other spreading code do not correlate and are not restored to the narrow band. In view of the desired signal, they thus appear as noise. The object is to detect the signal of the desired user from among a plural number of interfering signals. In practice, the spreading codes may partly correlate, and in such instances the signals of the other users make it more difficult to detect the desired signal, by distorting the received signal non-linearly. This interference caused by the users to one another is called multiuser interference.
Apart from multiuser interference, another factor that impairs performance in a cellular radio system is multipath propagation. In a typical cellular radio environment, a signal propagates from a transmitter to a receiver along many different paths. This multipath propagation results from the signal being reflected from surfaces, such as buildings, in the vicinity of the radio path. There is often no direct line of sight between the transmitter and the receiver, whereby the receiver does not receive one strong, directly propagated signal, but several reflected signal components that are of approximately the same power and have propagated along different paths. Since the paths have been different in length, the signals are slightly out of phase.
In conventional CDMA receivers intended for a multipath environment, so-called RAKE technology has been applied. A RAKE receiver comprises several correlators, each of which is able to synchronize with a signal component propagated along a different path, the received signal components being able to perform combination and detection in the receiver in an advantageous manner. The multipath propagated signal components can thus be utilized. A conventional RAKE receiver, however, is not able to cancel multiuser interference. For example, when multiuser interference is strong and the signal of one user is much stronger than those of the other users, the other users may be disconnected. This kind of situation may arise, for example, at a base station when one of the users is very close to the base station and the other users are further away. Without immediate power adjustment, the nearest user prevents the other users from being heard at the base station receiver.
One receiver solution operating in an environment with multiuser interference is a so-called decorrelating detector, which cancels multiuser interference by multiplying by the inverse matrix of the cross-correlation matrix of the spreading codes. Known decorrelating detectors, however, have been presented only on a Gaussian noise channel, and it has been difficult to apply them to a changing multipath channel, since changing of the correlation matrix in accordance with the changes on the channel has been complicated as regards computation. A decorrelating detector is discussed in greater detail in R. Lupas, S. Verdu: `Linear Multiuser Detectors for Synchronous Code-Division Multiple Access Channels,` IEEE Transactions on Information Theory, Vol. IT-35, p. 123-136, January 1989, and R. Lupas, S. Verdu: `Near-Far Resistance of Multiuser Detectors in Asynchronous Channels,` IEEE Transactions on Communications, Vol. COM-38, March 1990,